In the past, twisted pair cables were utilized in applications where data speeds reached an upper limit of about 20 kilobits per second. Recent advances in wire technology and hardware equipment have pushed the upper limit of twisted pair cable applications to about several hundred megabits per second.
Twisted pair technology advances have primarily focused on near end crosstalk. Both U.S. Pat. No. 3,102,160 and U.S. Pat. No. 4,873,393 teach the importance of utilizing pairs which are twisted with lengths of lay different from integral multiples of the lengths of lay of other paired conductors within the cable. This is done to minimize electrical coupling between paired conductors.
U.S. Pat. No. 5,015,800 focuses on another important issue of maintaining a controlled impedance throughout the transmission line. It teaches how impedance can be stabilized by the elimination of air gaps around a twisted pair embodiment through the use of a dual dielectric.
Several problems still exist which limit the use of twisted pair cabling. A primary concern is with the control of center to center conductor spacing. In a typical twisted pair cable, if pair one has a differentiation of only 0.002" in center to center conductor separation from pair two, a 6 ohm difference in average impedance can result. This is a fundamental reason why twisted pair cables have impedance tolerances of no better than +/-10%.
When two or more pairs of different average impedance are connected together to form a transmission line (often referred to as a channel), part of the signal will be reflected at the point of attachment(s). Reflections due to impedance mismatch ultimately causes problems with signal loss and tracking errors (jitter).
Prior attempts to control conductor spacing has been entirely for the purposes of stabilizing capacitance within a cable. It is well known in the industry that utilizing a cable with uniform capacitance between its pairs has the advantage of reducing crosstalk. U.S. Pat. No. 3,102,160 explains how equal and uniform capacitance can be achieved along a transmission line by simultaneously extruding dielectric over two conductors.
However, U.S. Pat. No. 3,102,160 did not recognize problems encountered with impedance mismatch at high frequencies. The impedance of the cable was of little importance provided the capacitance of each pair within the cable was relatively uniform. The problem is in that different cables can have uniform capacitances between their respective pairs and yet possess different average impedances.
To solve this problem, it becomes necessary not only to control the center to center conductor spacing of pairs within a particular cable, but to provide a consistent documented center to center conductor spacing requirement on all cables of a particular design. In this way, potential impedance mismatches between cable to cable connections will be held to a minimum. This improvement will ultimately allow more energy to be delivered to a receiving unit. Additionally, the signal will not be as distorted when compared to a typical twisted pair cabling structure due to decreased reflections along the channel.
Another problem with the U.S. Pat. No. 3,102,160 is with regard to insulated conductor separation. In order for the pairs of the said cable to be used with current LAN systems and connecting hardware, the adjoined insulated conductors must have the ability to be separated from one another for at least 1 inch along the length of the pair. The prior art provides no means for the separation of the two adjoined insulated conductors.
Generally in use today we have cables consisting of twisted pair groups, each group being formed from separate insulated conductors. These separate twisted pair cables can be effective in providing electrical energy in low frequency applications. These twisted pair cables have been used in applications ranging from telephone interconnect to LAN systems. The frequency range of these cables have been traditionally limited to about 10 MHz. With the advent of additional equipment such as media filters and signal regenerators, cables consisting of pairs which embody individually insulated conductors are beginning to run at speeds of several hundred MBps (Mega Bits per Second). However, this extra equipment can add subsequent cost to the overall system. As a result, many people still elect to install coax, which is generally regarded as a more electrically consistent cable media.
One reason why twisted pair cables are restricted in frequency is that they often have higher structural variation when compared to their coaxial counterpart. These variations can and will result in loss of energy via electrical reflections within the cable. The main cause for the increased variation is due to the elevated inconsistency of conductor to conductor spacing after twinning. This is especially evident with insulated conductors possessing poor concentricity. Additionally, increased variation of conductor to conductor separation can be a result of loosely twisted insulated conductors. This is because of varying air gaps which form between them.
Structural variations, such as those caused by less than desired concentricity within the insulated conductors of the twisted pair cause energy to be reflected back towards the source due to the subsequent changes in the impedance along the cable paths. Since the structural variations are cyclical along the transmission line, the impedance effect is additive, and what begins as a small discontinuity usually will turn into a major discontinuity. This reflected energy caused by structural variations is called return loss, and is considered lost power that is no longer useful to the system. Moreover, along with the return loss caused by the structural variations, the reflected wave can also be re-reflected at the source input, which may cause data errors at the receiving end.
Accordingly, it is an object of this invention to provide a twisted pair cable having a pair of insulated conductors joined along their length and twisted and said twisted conductors having a center-to-center distance varying over any 1000 ft. length of .+-.0.03 times the statistical average to reduce the structural variations normally associated with twisted pair cables and allowing more energy to be delivered to a receiving unit.
It is a further object of this invention to provide a twisted pair cable that allows for tighter tolerance of characteristic impedance, thereby reducing the potential for mismatch.
Accordingly, it is another object of this invention to provide a twisted pair cable with minimal structural variations to reduce the amount of reflected signal along the transmission line and approach the highly desired electrical uniformity of coaxial cable.
In accordance with these and other objects, a twisted pair cable is provided that can be used in high frequency applications. In one embodiment, the twisted pair cable has a pair of spaced central conductors surrounded by a dielectric(s) layer or insulation. The dielectric(s) layer is a pair of spaced cylinders longitudinally connected by an integral web. The conductors are substantially concentric with the dielectric layer and adhere to the inner wall of the dielectric layer to prevent relative rotation between the conductors and the dielectric layer.
The two dielectric layered conductors are interconnected by an integral solid webbing. The webbing preferably extends substantially the length of the wires and interconnects the diametrical axes of the dielectric layer over each conductor. In addition, preferably, the webbing has a thickness and width that are less than the thickness of the dielectric layer adjacent to the conductors. The dual conductor surrounded by the dielectric(s) layer is twisted to form a twisted pair cable. The variation in the distance between the centers of adjacent conductors, the center-to-center distances, along the twisted pair cable is very small. The center-to-center distance at any one point along the twisted parallel cable does not vary by more than .+-.0.03 times the statistical average of center-to-center distances measured along the twisted parallel cable.
Because the conductors are unable to rotate relative to each other and also are unable to form air gaps between adjacent insulated conductors, the structural variations are reduced. Thereby the return loss normally associated with twisted pairs is reduced. Additionally, the twisted pair cable allows for tighter tolerance of characteristic impedance, thereby reducing the potential for mismatch between successive cable runs.
In another embodiment of the present invention, single insulated conductors are affixed together substantially along their entire length by an appropriate adhesive or attached before the dielectric layers of adjacent wires are hardened. The adhesive is any appropriate dielectric adhesive for the conductor dielectric layer. Also, the twisted pair cable of our invention has an average impedance of about 90 to about 110 ohms when measured at high frequencies of about 10 MHz to about 200 MHz with an impedance tolerance of .+-.5% of a statistical average measured from randomly selected 1000 ft. cable of the same size taken from successive runs.
Our invention also permits the two attached (by web, adhesive or equivalent) insulated singles to be separated at a later time. Our insulated single conductors which are attached, have an adhesion strength of not more than 5 lbs. force. When being used in patch panels, punch down blocks, and connectors, it becomes necessary for the two singles to be segregated from each other. The spread can be up to one inch or more. With Twin-Lead type technology, the two wires cannot be uniformly detached--a distinct disadvantage when compared to our invention. It should also be noted that many connectors, such as the commonly used RJ-45 jack, require that the individual singles be uniformly round. With our invention, once the singles are detached, they will retain their roundness independent of each other.
The present invention and advantages thereof will become more apparent upon consideration of the following detailed description when taken in conjunction with the accompanying drawings.